The present invention relates to an electrophotographic printing apparatus, such as a laser beam printer or copying machine and methods of controlling the same. More particularly the present invention relates to a high-speed, high-density, long-service-life electrophotographic printing apparatus and methods of controlling the same.
Electric charging due to friction, corona static induction or by a corona charger is used in a conventional apparatus, as disclosed in Japanese Patent Publication JP-A-53-31136. A theoretical analysis of charging of a developer used in electrophotographic printing techniques is described in detail in R. M. Schaffert, "ELECTROPHOTOGRAPHY", published by THE FOCAL PRESS; and translated by Eiichi Inoue under the title "THEORY ON DEVELOPMENT OF XEROGRAPHY IMAGE" and published by Kyoritu Publishing, Section 5, pp. 329-347.
FIG. 9 shows printing by an electrophotographic apparatus used generally. The apparatus includes a charger 35, an exposure unit 40, a developing unit 65, a transfer unit 70, a fixing unit 80 and a cleaning unit 90. The developing unit 65 includes a carrier 12, toner 13, a toner density sensor 55. Toner 13 stored in a hopper 10 is supplied to the developing unit 65. The transfer unit 70 and the fixing unit 80 are disposed in the path 75 for recording paper. The mixed ratio of toner to carrier is measured by the toner density, sensor 55. The operation of supplying toner controls the toner density to be constant.
In the electrophotographic processing, part of the charged developer is lost in transference to the recording paper, the electric charges on most of the developer are cyclicly moved in the developing unit 6 and repeatedly used. By such cyclic motion, the developer cannot maintain its initial quantity of electric charges and the quantity of charges decreases. This is a so-called fatigue or deterioration of the developer. In these well-known examples, a decrease in the quantity of charges is predicted and a slightly excessive constant quantity of charges is supplied. In these known examples, an electrophotographic process is not found in which the developer in actual use is taken, its quantity of electric charges is measured, and the developer is recharged in accordance with the result of the measurement to control the quantity of electric charges.
In the prior art, a quantity of electric charges changes depending on the frequency of use of the developer and the ambient conditions under which the developer is used, and measures for keeping the quantity of charges constant are not provided. Other problems with the prior art are a change in the developer density, toner scattering and hence the shortening of the service life of the developer.